In packet switching systems, a large number of customers are typically connected to a main packet switching network via digital customer interfaces. Each interface accepts packets from the customer via low-speed transmission links and retransmits them to the switching network via a high-speed transmission link. In such packet switching systems, an administrative processor performs the administrative functions such as network usage billing and is associated with the main packet switching network. Each digital customer interface monitors and reports to the administrative processor the network usage of each of its customers. Customers are charged for network usage according to the number of packets they send and receive; the charge varies according to the time of day when each packet is sent. Due to the sensitivity of customers to overbilling, it is important that the real-time clocks in the digital customer interfaces be synchronized to the correct time of day, which is maintained by a real-time clock in the administrative processor. Such synchronization requires the interchange of messages containing the state of the real-time clocks in the administrative processor and the digital customer interfaces.
The most efficient manner to communicate real-time clock information is via the high-speed transmission links interconnecting the digital customer interfaces to the main packet switching network. When the transmission on the high-speed link is controlled by a more complex protocol such as X.25 (CCITT 1976, Amended 1980), a problem exists in that such a sophisticated protocol incorporates data for controlling the flow of packets under certain traffic conditions and particularly when the packet rate exceeds an arbitrary level. Flow control causes variable delays in the transmission of packets. The introduction of unpredictable packet delay makes an X.25 controlled transmission link unsuitable for the communication of clock synchronizing information since it is necessary to minimize and accurately predict packet transmission delay during real-time clock synchronization.
In prior art systems, this problem is solved by providing a separate communication channel between the digital customer interface and the administrative processor. Such a separate channel is not subject to the possibility of undesirable transmission delay due to flow control since it is not controlled by a sophisticated protocol. A dial-up telephone link has been used between the digital customer interface and the administrative processor for this purpose. Such an approach suffers from the disadvantages of dial-up operations, added control complexity, and higher system costs.